1. Field of the Invention
The present invention relates to an automatic spot soldering machine, and more particularly to an automatic soldering machine used for high volume assembly operations of components such as PC boards, electrical terminals, and the like.
2. Description of Related Art
Many attempts to develop general-purpose automatic spot soldering equipment have been made in the past, but these attempts have not been entirely successful. This is evidenced by the large number of types of machines and methods presently used such as induction, laser, resistance, open flame, hot air, molten solder, infrared, electrical resistance, and hot iron systems. Each method of heating the soldered parts and melting the solder and flux have advantages and serious disadvantages which exclude any specific type of machine for usage on a wide variety of parts.
The most prevalent systems-consist of a metal tip that is heated to a pre-selected high temperature by any convenient heat source such as an electrical resistance element.
Solders that are predominantly used typically melt at about 400xc2x0 F. Non-corrosive fluxes activate at 400xc2x0 F. to 600xc2x0 F. and rapidly decompose at temperatures over 600xc2x0 F.
The most convenient method of providing the solder and flux is in the form of a solder wire containing a core of rosin flux.
Solvents used in fluxes vaporize at about 200xc2x0 F. and do so quite explosively at temperatures over 600xc2x0 F. This splatters flux and solder particles adjacent to the tip. The solder tip temperature for fast and good saturation of parts is usually desired to be over 800xc2x0 F.
In spite of good vacuum systems, the solder/flux sputter and condensation of the flux vapors collect on the mechanical components that are in close proximity to the solder tip. The condensed flux is tacky when warm and solid when cold. This adversely affects the motion of mechanical components.
At solder tip temperatures over 600xc2x0 F., the solder coating on the metal tip periodically becomes removed, which seriously reduces the heat conduction and possibly produces defective parts. Recoating the tip with solder requires tip cleaning motion with a cleaning pad, which interrupts a production line sequence.
A typical cycle sequence for soldering consists of: 1) lower the hot solder iron on top of the part(s); 2) feed the solder wire point against the side of the hot solder tip, at the interface of the tip and part; 3) dwell in this position until the part(s) are heated to over 400xc2x0 F., which allows the flux to react and the molten solder to saturate the part(s); and 4) remove the solder iron to the initial position.
Another typical sequence is to: 1) feed the solder wire out between the part(s) and solder tip; 2) lower the solder tip onto the solder wire; 3) melt the solder (and flux) and continue the tip motion to allow the tip to press against and heat the part(s); and 4) dwell and then release the solder iron.
The present invention is designed to provide a low cost, low maintenance, general-purpose machine, which also eliminates the problems involved with other systems.
The cycle sequence of the solder iron tip motion, tip temperatures, tip velocity, solder wire feed rates, and solder wire feed pressures are precisely controlled for each application with considerations including the part size, solder wire size, cycle speed, dwell time between parts, factory temperature variations, line voltage fluctuations, and material variations. These adjustments can only be optimized by an experienced person with access to proper tools and equipment.
One example according to the teachings of this invention provides a non-adjustable machine that is preset at the solder machine factory for a specific part.
A second example according to the teachings of this invention provides an automatic spot solder machine with adjustable controls limited to the solder wire feed length and cycle speed.
A third example according to the teachings of the present invention includes means to heat the tip from 500xc2x0 F. to 700xc2x0 F. within 0.75 seconds or less, including means to cool the tip from 700xc2x0 F. to 500xc2x0 F. within 1.5 seconds or less.
The rapid cooling and heating of the solder iron tip are required to maintain a reasonable high production rate, which also provide many variations of the basic cycle sequence. Also, the cool tip prevents the tip surface from oxiding and losing its solder coating when not in use. However, if some area of the tip loses its coating, a low temperature melting of the solder and flux allows the tip to become re-coated with solder. This eliminates the need for a tip xe2x80x9ccleaningxe2x80x9d cycle.
An exemplary cycle utilizing the teachings of the present invention can be: 1) the solder iron tip is normally at a temperature of 300xc2x0 F. to 500xc2x0 F. when idle; 2) feed the solder wire out to position the front section between the tip and part(s); 3) simultaneously lower the tip onto the solder wire and heat the tip to a temperature of 400xc2x0 F. to 600xc2x0 F.; 4) melt the solder wire and flux at this low temperature, which reduces or eliminates splatter of flux and solder; 5) increase the tip temperature to 600xc2x0 F. to 1200xc2x0 F., for a rapid saturation of solder into the part(s); and 6) return the tip to its original position and simultaneously allow the tip to cool to a temperature of 300xc2x0 F. to 500xc2x0 F. in time for the next cycle.
Another exemplary cycle utilizing the teachings of the present invention can be: 1) the solder iron tip is normally at a temperature of 300xc2x0 F. to 500xc2x0 F. when idle; 2) lower the tip onto the part(s) and simultaneously heat the tip to a temperature of 500xc2x0 F. to 700xc2x0 F.; 3) feed the solder wire/flux into a hole on the side of the solder tip, where a connecting hole on the bottom surface of the tip allows the molten solder and flux to exit the tip and onto the part(s) (see FIG. 3); 4) increase the tip temperature to 600xc2x0 F. to 1200xc2x0 F., for a rapid saturation of solder into the part(s); and 5) return the tip to its original position and simultaneously allow the tip to cool to a temperature of 300xc2x0 F. to 500xc2x0 F. in time for the next cycle.
A fourth example according to the teachings of the present invention includes a solder iron tip with a cross hole for feeding the solder wire into the hole. This allows the solder/flux to melt within the solder tip, which eliminates all splatter of flux and solder. Exiting the molten solder/flux through a hole at the bottom of the tip places the solder/flux exactly where the part(s) are located. (See FIG. 3).
The objectives in feeding the wire through the tip include: 1) prevent splatter of flux and solder; 2) deposit the solder/flux precisely; 3) reduce the decomposition of the flux by reducing the area and contact time of the flux on the hot solder tip, which also improves the solder saturation into the parts and assists in maintaining a coated solder tip; and 4) create a high-pressure extrusion of the molten solder/flux out of the solder tip, onto the part(s).
Many previous attempts by others and this inventor to provide a solder wire feed through a hole in the solder tip were unsuccessful. Large diameter solder/flux wire over 0.125 inches, which is seldom used in high production, is relatively easy to process by this method. Smaller diameters are possible only with a precise design and control of many inter-related variables.
These variables are: temperature of the solder tip; diameter and feed rate of the solder wire; diameter, length, and thermal conductivity of the solder tip; size and length of the entrance hole; cycle speed; and timing. It is desired to feed the solder wire through the hole without any blowback. The entrance hole may be larger or smaller than the solder wire diameter. If the hole is smaller than the wire, the wire may be pushed in with a high force and high speed, which shaves or melts the outside surface of the wire. If the tip temperature is hot enough, the wire surface can be melted at any reasonable speed. If the feed rate is too slow for any given temperature, the outside surface of the wire melts to a depth, which allows blowback (See FIG. 4).
If the diameter of the hole is larger than the wire diameter, which is the more practical method to control the other parameters, then the sealing of the space between the wire and hole wall is made by the solidification of molten solder. The liquid solder blows back into the space, which rapidly cools when in contact with the inwardly moving wire. This effectively increases the wire diameter to the dimension of the hole, leaving no space for a blowback (See FIG. 5). In this configuration, the length of the hole becomes a significant factor. If the length of the hole is very short, then the solder/flux blows out before it has a chance to cool, even if the hole clearance is very small. If the hole length is very long, then the hole size may be very much larger than the wire size, especially if the hole is a long tube with a cold (less than the melting temperature of the wire) entrance opening, in which case the feed rate may be fast or very slow (See FIG. 6).
In all configurations, the maximum feed speed is limited to the speed at which the wire point melts. The melting speed is a function of the wire diameter, length of the hole, tip temperature, feed pressure and the thermal conductivity between the hole and the wire surface.
Another design variation, which decreases the criticality of the temperature, feed rate, hole size, and length is to plate the hole wall with a non-solderable material such as chromium plating or ceramic. The non-solderable surface will substantially reduce the thermal conductivity. This requires a hole larger than the solder wire diameter. If the other design parameters are such that a slight blowback occurs, then the flux decomposes and the residue adheres to the wall of the hole, which reduces the diameter of the hole. Also, the non-solderable hole wall is a poor heat conductor. Therefore, the incoming cold solder wire quickly solidifies the outward moving solder/flux.
A fifth example according to the teachings of the present invention includes a solder wire feeder and an accurate wire guide mechanism. The solder wire is ductile with a low beam strength, cold flows, not always straight and free of kinks and quite often becomes coated with flux. Also, the guide and feeder mechanism require accommodation for heat and flux contamination. One disclosed example consists of a rigid guide rail, which is open on the top surface, in the form of a groove. The solder wire nests within the groove and is held down with a spring, which is positioned close to the end of the rail. The spring maintains a pressure against the wire, which keeps the wire straight and deflects whenever irregularities on the wire feed through (See FIG. 2).
To prevent buckling of the free end of the wire, between the end of the rigid rail and solder tip, the end of the rigid rail is positioned within 16 diameters of the solder tip. It is anticipated that this rail could extend up to the solder tip or, if fabricated out of a low heat conductive material such as ceramic, it could be an integral part of the solder tip (See FIG. 6). All disclosed designs require the feeder mechanism to be small to provide access of the solder tip to various parts which are soldered, be tolerant to contamination from flux and solder particles, and have a low inertia to minimize the forces required for a rapid feed and pull back of the wire. Also, the infeed of the solder wire is occasionally obstructed or encounters a hesitation. This requires that the feeder apply a forward force on the wire that is as high as possible but less than the buckling force of the wire.
The high solder wire feed force, combined with a good seal between the solder wire and the in-feed hole on the solder tip, creates a high out-flow pressure of the molten solder. The solder wire feeding into the hole acts like a piston.
The disclosed machine includes a cam actuated feed finger consisting of a lever with a small point which imbeds and engages the solder wire. The configuration of the point and the depth of penetration determine the forward force that is applied by the lever onto the wire. If an obstruction prevents the wire from feeding forward at the velocity of the feed finger, the point cuts a path in the top surface of the wire, which imparts a predetermined, maximum, constant forward force on the wire (See FIG. 2). The constant force on the wire is imparted to the wire at the point of engagement. Therefore, all feed mechanism inertia forces are eliminated. It is anticipated that other more complicated methods of applying a constant force are possible many with compromises on the basic requirements.
The disclosed machines provide a relatively constant force, and are relatively simple in construction, contamination tolerant, small profile, and low cost.
The feed finger is actuated by a cam motion, which provides a simple, high force, high velocity drive system for the feeder, which is always in exact timing with the tip motions.
The combination of the high force, high velocity, and close fitting guide rail causes the soft, gummy solder wire to gall (stick) within the guide groove. A very light coating of oil is required on the solder wire. Most solder wire manufacturers do not provide lubricated wire.
A sixth example according to the teachings of this invention includes an oil pouch having a rectangular piece of open cell neoprene, vacuum filled with oil, and sealed in a plastic bag, which provides a clean method of handling this item (See FIG. 1, reference number 17). It is installed into the machine system by piercing the solder wire through the plastic and foam. This item costs on the order of one dollar and replaces a previous oiler and wiper system that cost approximately one-thousand five-hundred dollars. A seventh example according to the teachings of the present invention includes a means to electrically heat the tip with a low voltage and amperage greater than 300 amps, at 10 to 100 Hz. The high amperage alternating current creates an electrical/magnetic field in the molten solder, tip, and parts, which vigorously vibrates the molten solder. This improves the saturation of the solder into the parts. The input power of 60 Hz AC is ideal for this function.
The high amperage AC also repels molten solder from the solder tip. Therefore, a short burst of high current at the instant when the tip is removed from the part repels most of the excess solder that normally adheres to the tip. This action eliminates a sharp residual point of solder (known as an icicle) from forming on the surface of the part. Another advantage of this repulsion effect is to eliminate any splash of solder balls during the tip retraction. When the tip separates from the part, the molten solder strings out into a bridge between the tip and part. When the string breaks, a section of the string sometimes breaks away into a small ball and flings out. The repulsion effect of the solder tip eliminates any bridging effects.
An eighth example according to the teachings of the present invention includes a solder cycle wherein a short pulse of alternating current is applied to the solder tip simultaneously with the upward motion of the solder tip.
A ninth example according to the teachings of this invention consists of a solder tip holder, which is required to be moveable, stiff, precision, and tolerant to contamination. These requirements are provided for by placing the solder tip holder on a support bracket, which is then pivot mounted on pre-loaded ball bearings. The pivot bearings are located at some distance away from the solder tip (See FIG. 1, reference number 14).
A tenth example according to the teachings of the present invention includes a cam drive, which powers substantially all of the mechanical motions (See FIG. 1, reference number 12).